Most commercially available position encoders are based on glass scales which are transilluminated with an array of secondary gratings. The shadow of the gratings, forming Moire effect is analyzed with photodiodes giving information about relative position. For example such an encoder [Model LS 106, J. Heidenhain GmbH, Traunreut, Germany] provides a resolution down to 0.5 mm. Many systems, like proposed by Ishizuka [U.S. Pat. No. 5,661,296] and Pettigrew [U.S. Pat. No. 4,776,701] involve separation of diffractive orders obtained from first diffraction grating which are then brought together and interfered giving fringe pattern which analyzed by photodetector. More advanced high resolution encoders, for example [Model L-104, Canon USa Inc., New York, N.Y. 11042], are based on the diffraction of an illuminating beam at a grating and detection of the interference pattern of selected diffraction orders. Position encoder of Remijan [U.S. Pat. No. 4,395,124] and [U.S. Pat. No. 4,542,989] detects a fringe pattern, created by diffraction of zero, plus first and minus first orders from phase diffraction grating. Light from He-Ne laser is collimated and focused at a focal point at a distance from grating. The spherical wave illuminates the grating, designed and fabricated to diffract equal intensity 0 and +/-1 orders. Zero order cone interferes with plus and minus cones separately giving fringe patterns. If the grating is moved in a plane that is perpendicular to the direction of the fringes, all the fringes appear to slide in the plane of photodetector, providing a possibility to encode a position. Mitchell [U.S. Pat. Nos. 5,486,923, 5,646,730, and 5,559,600] uses phase grating with minimized zero order. A poly-phase periodic detector is spaced close to the grating so that each detector phase or element responds principally to interference between the positive and negative first orders diffracted from the grating without intermediate reflection or magnification.
Three types of optical encoders, which use fibers, are known. The first type uses fibers only as a light delivery device. The advantage of using optical fibers in existing encoders was that fibers allow the electrical elements of the light source and decoding circuitry to be remotely located from the code plate. For example, Yeung [U.S. Pat. No. 4,767,164] disclosed rotational fiber optic encoder. The system utilizes optical fiber to transmit light to a pair of interrupter disks, and to collect light reflected from disks. Both disks are provided with alternating reflected and transmissive parts. Decoding circuitry is provided to convert the modulated component of the light signal to an electrical signal that represents the rotational speed of the wheel. Lenox [U.S. Pat. No. 4,240,066] described the cylindrical encoder for use with fiber optics, which contains of transmitting head of fibers, receiving head of fibers and code plate with windows that can slide between the heads. This enables the optical signals to be picked up reliably, but still requires bundle of individual fibers for transmitting head and bundle of fibers for receiving head, that makes system very complicated. Urbanik [U.S. Pat. No. 4,442,423] describes fiber optic position sensor, which employs 4 fibers for transmitting and 4 for receiving light passed through code plate with windows. Senuma [U.S. Pat. No. 5,498,867] suggested wavelength and time-division multiplexing to distribute light pulses to different collimators in encoder, and thus employ only one light source and one optical fiber. The limitation of position resolution (tens of micron) is due to code plate technique.
The second type of fiber encoders uses fibers for measuring strain in material. Zimmerman B. D et. al [U.S. Pat. No. 5,649,035] disclosed fiber optic sensor with the two reflective markers. This fiber is attached or embedded into the structure of material. An optical signal is input into the fiber and reflected at reflective markers at predetermined positions in the fiber. The time delay of the signals received back is analyzed to calculate strain in the structure. Bieren K. et al [U.S. Pat. No. 5,201,015 ] suggested conformal fiber optic strain sensor, where fiber is attached between two points of connection under tension. An interferometer is formed in the tensioned portion of the fiber. The sensor is mounted to a surface and changes in interference patterns output by the interferometer are monitored to measure strain in the surface.
The third type of fiber encoders, for example one has been disclosed by Udd E. et. al. [U.S. Pat. No. 5,397,891] uses fiber gratings to sense strain that can vary the spacing between the lines of the grating to vary center wavelength of the reflection or transmission spectra. Strain can also change the relative distance between two gratings written in one fiber, each at a different end, and thus change the resonance build up of light at certain wavelengths. The last approach was demonstrated by Glenn W. et. al. [U.S. Pat. No. 4,950,883].
All above-mentioned types of fiber encoders have a limitation of position encoding length.
At some extent more advanced approach has been chosen by Wanser [U.S. Pat. No. 5,661,246], which uses bending characteristics of fiber. His Fiber Optic Displacement sensor measures the distance between a fixed point and movable location using light loss characteristics of bent optical fiber. The sensor takes advantage of the specific shape that the fiber assumes upon changing the distance between the two attachment points. Reproducibility of this method requires well defined boundary conditions of the holders of the fibers. The effect is highly non-linear with the largest contributions in the regions of smallest bend radii and fastest change of bend radii. The sensitivity of this method is about 1 mdB/mm and since values like 10 mdB can be measured, position resolution of this method does not exceed 10 mm. Moreover, it is obvious, that this method will not work for very long length of fiber.
To date, all diffraction-based linear or rotational optical encoders have been made on flat plates (glass or other material) and thus:
1. Their sizes are limited by substrate size availability (maximum about 20"). PA1 2. The price of such substrates grows almost exponentially with their sizes. PA1 3. The surface of the motion controlled elements (we don't consider strain control, but motion) has to be plane (non-conformal). PA1 1. Fiber Bragg Grating-phase grating-refractive index modulation grating encrypted in the core of the fiber; PA1 2. Fiber Surface Relief Grating-phase grating, created PA1 3. Fiber Amplitude or Amplitude-Phase Grating--Grating, made with opaque (metal) pattern layers on fiber jacket, fiber cladding layer, or fiber core surfaces.
Few known optical encoders, based on flexible metal tapes, can't provide high positioning accuracy due to non-flat surface and strong temperature dependence.